The investigation intends to build an instrument designed to help rapidly analyze mixed associations of the type nA+ mB yields AnBm. This unit will be composed of an HPLC gradient mixing device, a flow-through low angle laser light scattering photometer, and a dedicated computer system to both control the experiment and to collect and analyze the results. The instrument will be able to quickly generate a three dimensional surface, having as coordinates the two total protein concentrations and the resulting weight-average molecular weights, and to analyze this surface in terms of association constants and stoichiometries. The protein system to initially be examined is the interaction between Factor B and the different forms of protein C3 in the alternative pathway of complement. This pathway provides a mechanism of (largely) non-antibody dependent immune response. This interaction is also directly involved in the neutropenia response of patients exposed to "unconditioned" dialysis membranes and in lowered immune effectiveness after artificial materials are implanted. Products resulting from these associations are also implicated in decreasing secretion of B cell antibodies, activating suppressor T cells, inhibiting helper T cells and suppressing T cell proliferation. The Factor B, C3 interaction has been directly implicated in the biochemical problems of anorexia nervosa. Reinforcing the importance of this protein system is the observation that, despite the many complement component genetic deficiencies known, no one yet has been able to find a case where Factor B is missing. The proposed instrument is important in its own right because it will allow other investigators to examine other physiologically important mixed associations between macromolecules. The investigation centered on the interaction of Factor B with the different forms of C3 (and in the longer term, other complement factors such as H and P) is important for its attempt to understand the relative roles of the competing interactions in this segment of the immune regions.